The present invention generally relates to a phosphor, an optical device including the phosphor, and a display device including the phosphor.
A phosphor represented by chemical formula 4:(Sr(1-x-y)CaxBay)(Ga(1-m-n)AlmInn)2S4:Eu  [Chemical formula 4]
known as a thiogallate phosphor has a spectrum width in the green light region smaller than that of a phosphor (ZnS:Cu, Al) used in an electron-beam-excitation light-emission device, such as a cathode-ray tube, and thus, has a satisfactory color purity, as shown in FIG. 10 showing the relationship between the wavelength and the luminescent intensity (for example, see Journal of Luminescence Vol. 111, pp. 147-158, (2005)).
In general, as the emission center wavelength approaches the wavelength at which the luminous efficacy is the maximum in the color region and as the width of the spectrum increases, the luminance of the phosphor increases. Therefore, the luminance of the thiogallate phosphor depends on whether the spectrum center is close to a wavelength of 555 nm at which the luminous efficacy is the maximum or not, and the width of the spectrum.
However, for example, as is apparent from the fact that the emission spectrum of pure green light has a center wavelength of about 520 nm and a small width, a phosphor having satisfactory color purity inevitably tends to have a low luminance. For example, it is known that the fluorescent luminance obtained by electron beam excitation of the thiogallate phosphor is lower than that of the ZnS:Cu, Al phosphor by about 23%.
On the other hand, in display devices including organic electroluminescence (EL) elements, which have recently attracted attention, a structure combining a blue luminescent layer with a color conversion layer has been studied. In the known display devices, since the durability of an organic dye constituting the color conversion layer is not satisfactory, the efficiency of the color conversion markedly decreases (with time) with continued use of the device.
To overcome this problem, a method of using a color conversion layer including an inorganic phosphor having a high durability has been proposed (for example, see Japanese Unexamined Patent Application Publication No. 2002-151268). According to this technique, the above-described degradation with time can be suppressed by using an inorganic phosphor as the color conversion layer. However, even this structure still has an insufficient luminance.
The low luminance is disadvantageous not only in optical devices, such as an illuminator, but also display devices that include a phosphor, and thus an improvement in the luminance is desired.
In general, there is a certain tendency in the absorption wavelength range of a phosphor, and the wavelength of the irradiating light for exciting the phosphor is selected according to the absorption wavelength range. However, in some display devices, not only the emission from the phosphor but also the above irradiating light itself is used for a part of the pixels. Thus, the selection of the irradiating light has also been restricted.